in6_if2idlen: treat bridge(4) interfaces like other Ethernet interfaces

bridge(4) interfaces have an if_type of IFT_BRIDGE, rather than
IFT_ETHER, even though they only support Ethernet-style links.  This
caused in6_if2idlen to emit an "unknown link type (209)" warning to
the console every time it was called.  Add IFT_BRIDGE to the case
statement in the appropriate place, indicating that it uses the same
IPv6 address format as other Ethernet-like interfaces.